The present invention relates to color selecting electrodes for cathode-ray tube and, more particularly, to a color selecting electrode for cathode-ray tube which is prevented from deterioration of characteristics such as color deviation even used for a long time thereby assuring improved reliability.
It is known that color selecting electrodes for cathode-ray tube are classified into two types; one is the so-called shadow mask type and the other is the so-called aperture grille type.
The latter includes a frame having a pair of opposed first supports and a of opposed second supports extending as crossing the first supports, and grid elements disposed on the pair of first supports at a fixed pitch and stretchedly bridging the first supports. With such a color selecting electrode, color selection is made when an electron beam passes through a slit defined between adjacent grid elements.
A conventional frame for use in the aperture grille type electrode is formed by joining the first supports to the second supports by arc welding, the first and second supports being both made of a low carbon alloy steel, or the first supports being made of stainless steel SUS403 and the second support made of chromium molybdenum steel SCM415. The above-mentioned materials for the first supports are cold drawn, annealed and then formed into a predetermined shape, while the materials for the second supports are cold worked and formed into a predetermined shape. After welding, the frame is annealed to remove residual stress existing therein, machined at required portions thereof including portions on which the grid elements are to be stretchedly disposed, and further subjected to machining so as to withstand the tension of the grid elements.
The grid elements are formed by etching a steel sheet obtained by cold working a very low carbon steel to harden it as having a tensile strength of about 70 kg/mm.sup.2 to about 80 kg/mm.sup.2. A set of such grid elements is a so-called aperture grille assembly (hereinafter referred to as "aperture grille").
In turn, the aperture grille is subjected to seam weld to the pair of the first supports under pressurized conditions such that compressive stress works on the pair of second supports, or in other words the pair of first supports are made close to each other. When the pressurization against the frame is relieved, the restoring force of the frame produces tension on each grid element. The aperture grille in such a state will be hereinafter referred to as "stretched mask". Since tension is thus exerted on the grid elements, the aperture grille type electrode is capable of absorbing an expansion of the grid elements due to a rise in the temperature thereof caused by incidence of electron beam on the grid elements. The stretched mask is further subjected to heat processes including a bleckening (steam treatment, or oxidation in exothermic gas) at about 450.degree. C. to about 470.degree. C. for about 20 minutes and a glass sealing treatment at a temperature substantially equal to the temperature for bleckening, thereby forming a cathode-ray tube.
As described above, the process for manufacturing the conventional stretched mask is performed under severe conditions where the mask is exposed to high tensile stress and high temperatures. Specifically, each of the grid elements initially experiences at end portions thereof a tension of about 50 kg/mm.sup.2 (hereinafter referred to as "initial tension"), and the frame also experiences a bending stress of about 10 kg/mm.sup.2. Further, the stretched mask is subjected to a heat treatment at about 450.degree. C. to about 470.degree. C. for about 20 minutes. A tension at end portions of each grid element after the heat treatment (hereinafter referred to as "final tension") decreases because of a relaxation phenomenon associated with creep. As disclosed in, for example, Japanese Unexamined Patent Publication No. 249339/1987 at page 204 second column line 10 to third column line 9, efforts to enhance the durability against creep of an aperture grille are made such as addition of nitrogen to a very low carbon steel. However, the final tension becomes as small as about a half of the initial tension of about 50 kg/mm.sup.2.
Further, Japanese Unexamined Patent Publication No. 276137/1990 discloses at page 1 left column lines 8 to 11 an art of mitigating relaxation wherein a metal member having a thermal expansion coefficient larger than that of resilient supports (second supports) is fixed only on the side opposite to the side on which stretched bridges of grid elements are disposed to reduce the tension which is generated on the grid elements during a heat process, thereby mitigating relaxation.
With regard to the shadow mask type electrode, on the other hand, there is disclosed in, for example, Japanese Unexamined Patent Publications Nos. 42838/1986 and 68650/1975, an art of alleviating a color deviation by making a shadow mask comprised of a material having a low thermal expansion coefficient. However, unlike the aperture grille type electrode having bridges stretchedly provided of grid elements, this type of electrode has a mask which is not stretched. Therefore, there is no disclosure on the relationship between the tension of the grid elements and the thermal expansion coefficient thereof.
With the conventional stretched mask, a significant decrease in the tension on the grid elements is inevitable due to relaxation thereof and, hence, it is impossible to keep a desired tension distribution of the mask stably under long-time use conditions. Therefore, the conventional mask involves a problem that degradation of color selecting characteristics thereof is likely with an occurrence of color deviation.
Further, the first supports of the frame is likely to expand depending on the use condition because of a rise in the temperature of the cathode-ray tube whereby grid elements in end portions of the aperture grille are possible to be moved outwardly. This results in a problem of color deviation.
It is, therefore, an object of the present invention to overcome the problem of degradation in color selecting characteristics such as causing the color deviation because of thermal deformation of the aperture grille and to provide a color selecting electrode of high performance for cathode-ray tube in which a decrease is minimized in the tension of grid elements when the aperture grille is subjected to a heat process to assure a stabilized tension.
Another object of the present invention is to provide a color selecting electrode of high performance for cathode-ray tube which is substantially free from the color deviation due to deflection of the grid elements in the horizontal direction (the direction orthogonal to the direction in which the grid elements extend).
Yet another object of the present invention is to provide a color selecting electrode wherein grid elements having a low thermal expansion coefficient are used to reduce thermal expansion thereof in their stretching direction, so that the initial tension on end portions of each grid element can be set low enough to reduce relaxation thereby obtaining a desired tension stably even after the required heat process.